1. Field of the Invention
The present invention relates to a lithium secondary battery equipped with a cathode containing a cathode active material capable of intercalating and deintercalating lithium ions, an anode containing an anode active material capable of intercalating and deintercalating lithium ions, and a non-aqueous electrolyte, and it particularly relates to an improvement of the cathode active material.
2. Background Art
As lightweight and high capacity batteries for use in portable electronic and telecommunication devices and the like such as compact video cameras, cellular phones, and portable personal computers, recently put into practical use are lithium second batteries employing a carbon material capable of intercalating and deintercalating lithium ions as the anode active material and a lithium-containing transition metal oxide such as a lithium-containing cobalt oxide (LiCoO2) or a lithium-containing nickel oxide (LiNiO2) as the cathode active material.
However, although lithium-containing transition metal oxides such as lithium-containing cobalt oxide (LiCoO2), lithium-containing nickel oxide (LiNiO2), and the like possess a high battery capacity, they suffered a problem that they have low thermal stability in charged state; moreover, cobalt and nickel, which are the raw material, are expensive, and their reserve is limited. Accordingly, there is proposed a lithium secondary battery using a lithium-containing manganese oxide (LiMn2O4) having spinel type crystal structure as the cathode active material material. The lithium-containing manganese oxide (LiMn2O4) is one of the promising materials for use as the cathode active material material for a lithium secondary battery, in that manganese used for the raw material is abundant in resources and is inexpensive, and that it yields a high thermal stability in the charged state, such that it can increase the safety of the battery.
However, although the lithium-containing manganese oxide having the spinel type crystal structure (LiMn2O4: which is referred to hereinafter as xe2x80x9cspinel type lithium manganatexe2x80x9d) has excellent thermal stability, it still had problems concerning the battery capacity and the charge-discharge cycle characteristics. More specifically, since a spinel type lithium manganate contracts on charging and expands on deintercalating, the electrode suffers a change in volume with progressive charge-discharge cycles. Accordingly, the active material particles undergo dissociation due to the change in volume, and this presumably causes a drop in collector efficiency. On the other hand, a lithium-containing cobalt oxide (LiCoO2: which is referred to hereinafter as xe2x80x9clithium cobaltatexe2x80x9d) undergoes expansion on charging and contraction on deintercalating.
In the light of such circumstances, in Japanese Patent Laid-Open No. 171660/1992 is proposed to use a mixed cathode active material containing mixed therein a spinel type lithium manganate which contracts on charging and expands on deintercalating, and a lithium cobaltate which expands on charging and contracts on deintercalating.
By using a cathode comprising spinel type lithium manganate and lithium cobaltate in mixture, according to Japanese Patent Laid-Open No. 171660/1992, there can be realized a battery increased in capacity as compared with a case using spinel type lithium manganate alone, and a battery further improved in thermal stability as compared with a case using lithium cobaltate alone.
Concerning spinel type lithium manganate, the amount of intercalating and deintercalating lithium ions per unit mass, which is directly related to the battery capacity, is smaller than that of lithium cobaltate. Hence, in case of using a cathode material comprising spinel type lithium manganate and lithium cobaltate in mixture, there occurred a problem that the battery capacity is decreased as compared with the case using lithium cobaltate alone. Thus, measures on suppressing the drop in capacity has been considered by increasing the bulk density of the electrode retaining the active material of this type.
However, since lithium cobaltate consists of platy particles, the particles tend to show high orientation, and, in case the bulk density is increased, the lithium cobaltate particles become oriented in parallel with the collector. This leads not only to a decrease in the penetration of electrolyte, but also to a hindrance in maintaining the presence of crystallographic planes through which occlusion and discharge of lithium ions take place. Accordingly, lithium cobaltate suffered problems of decreasing load characteristics such as high rate discharge properties in case of increasing bulk density of the electrode.
The invention has been made with an aim to overcome the aforementioned problems, and an object thereof is to provide a lithium secondary battery, which, even in case a mixed cathode active material comprising mixed therein spinel type lithium manganate and lithium cobaltate is used, it still is capable of yielding improved load characteristics such as high rate discharge properties by optimizing, not only the bulk density of the cathode mixed agent, but also the mean particle diameter of both active materials, thereby suppressing the lithium cobaltate particles from being oriented.
In order to achieve the object above, the cathode for use in the lithium secondary battery according to the invention comprises, retained on a cathode collector, a cathode mixed agent based on a mixed cathode active material comprising a mixture of lithium cobaltate and a spinel type lithium manganate, wherein the mixed cathode active material contains lithium cobaltate at a mass ratio X in a range of not lower than 0.1 but not higher than 0.9; the cathode mixed agent is retained on the cathode collector in such a manner that the bulk density Y (g/cm3) should fall in a range not lower than 0.5X+2.7 but not higher than 0.6X+3.3; and the mean particle diameter of the spinel type lithium manganate is larger than the mean particle diameter of the lithium cobaltate. The bulk density Y for the cathode mixed agent herein signifies the mass of the mixed agent per unit volume of the cathode excluding the volume of the cathode collector.
Since spinel type lithium manganate is lower in electron conductivity as compared with lithium cobaltate, if the content of lithium cobaltate should be too low, the electron conductivity of the mixed cathode active material decreases as to lower the load characteristics such as the high rate discharge properties. On the other hand, if the amount of addition for spinel type lithium manganate should be too small, suppression on the orientation of lithium cobaltate particles results insufficient as to impair the load characteristics such as the high rate discharge properties. Accordingly, the mixed mass ratio of the spinel type lithium manganate is preferably set at 0.9 or lower but not lower than 0.1 (i.e., 0.1xe2x89xa6Xxe2x89xa60.9, where X represents the mass ratio of spinel type lithium manganate).
In case the bulk density of the cathode mixed agent based on the mixed cathode active material of lithium cobaltate and spinel type lithium manganate (more specifically, the cathode mixed agent comprises a mixed cathode active material, an electrically conductive agent, and a binder) is too low, the load characteristics such as the high rate discharge properties decreases due to reduced electric contact among the active material particles present in the cathode mixed agent. In case the bulk density of the cathode mixed agent is too high, on the other hand, destruction occurs on the particles of the spinel type lithium manganate due to the excessively high pressure applied to the mixed cathode active material on forming an electrode by applying an extremely high pressure. This results in the decrease of the load characteristics such as the high rate discharge properties due to the failure of preventing the lithium cobaltate from being oriented. Accordingly, as a result of extensive experiments performed, it is found that the cathode mixed agent is preferably retained on the cathode collector in such a manner that the bulk density Y thereof should be set as such to satisfy the relation of 0.5X+2.7xe2x89xa6Yxe2x89xa60.6X+3.3.
In the case above, if the mean particle diameter of spinel type lithium manganate should be smaller than that of lithium cobaltate, it is found that lithium cobaltate particles tend to show an orientation in parallel with the collector when packed at a high bulk density. As a result, the crystallographic planes through which the occlusion and discharge of lithium ions take place become less present on the surface of the electrode, and the permeability of the electrolyte decreases. These lead to a decrease in the load characteristics such as the high rate discharge properties. Accordingly, the mean particle size of the spinel type lithium manganate should be set higher than the mean particle size of the lithium cobaltate. In this manner, the lithium cobaltate particles are suppressed from being oriented in parallel with the collector, thereby leading to an improved permeability of the electrolyte and to the improvement of load characteristics such as the high rate discharge properties.
Then, in case the ratio of the mean particle diameter (B/A) of the spinel type lithium manganate and the lithium cobaltate is in a range of 1.5xe2x89xa6B/Axe2x89xa68.0, the compressive force can be properly dispersed among the spinel type lithium manganate and the lithium cobaltate particles even in case a high compression is applied to realize high bulk density for the formation of the electrode. Thus, the orientation of lithium cobaltate particles can be suppressed as to improve the load characteristics such as the high rate discharge properties. More preferably, the load characteristics can be further improved in case the ratio is set in a range of 2.0xe2x89xa6B/Axe2x89xa65.0.
In case the mean particle diameter ratio of the spinel type lithium manganate and the lithium cobaltate (B/A) is fixed, a spinel type lithium manganate with a mean particle diameter smaller than 6 xcexcm requires a higher compressive force in compressing the cathode mixed agent to a predetermined density, because the mean particle diameter for both spinel type lithium manganate and lithium cobaltate becomes smaller.
In this case, a further compressive force is applied to lithium cobaltate as a result, and this facilitates the orientation of lithium cobaltate particles as to lower the load characteristics such as the high rate discharge properties. On the other hand, spinel type lithium manganate with a mean particle diameter exceeding 40 xcexcm increases the mean particle diameter of both spinel type lithium manganate and lithium cobaltate. Then, the surface area of the both particles decreases as to reduce the reaction area in contact with the electrolyte, thereby leading to a decrease in the load characteristics such as the high rate discharge properties. Accordingly, the mean particle diameter of spinel type lithium manganate B (xcexcm) is confined, preferably, in a range of 6 xcexcmxe2x89xa6Bxe2x89xa640 xcexcm, and more preferably, in a range of 10 xcexcmxe2x89xa6Bxe2x89xa630 xcexcm.
Concerning the spinel type lithium manganate to employ in the invention, similar results are obtainable so long there is used such having a compositional formula of Li1+XMn2xe2x88x92YM2O4 (where M represents at least one type of element selected from the group consisting of B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Co, Ni, Cu, Al, In, Nb, Mo, W, Y, and Rh; 0.54xe2x89xa6((1+X)+Z)/(2xe2x88x92Y)xe2x89xa60.62; xe2x88x920.15xe2x89xa6Xxe2x89xa60.15; Yxe2x89xa60.5; and 0xe2x89xa6Zxe2x89xa60.1). Among them, however, particularly superior high temperature characteristics (e.g., cycle life characteristics at high temperature, storage characteristics at high temperature, and the like) can be obtained by preferably employing those of Mg added systems or Al added systems.
As the lithium cobaltate to employ in the invention, similar results are obtainable so long there is used such expressed by a compositional formula of LiCo1xe2x88x92XMXO2 (where M represents at least one type of element selected from the group consisting of B, Mg, Ca, Sr, Ba, Ti, V, Cr, Fe, Ni, Cu, Al, In, Nb, Mo, W, Y, and Rh, and 0xe2x89xa6Xxe2x89xa60.1). Among them, however, particularly superior discharge characteristics are obtained by preferably using those of Cr added systems, Mn added systems, Al added systems, or Ti added systems.